There is conventionally known a method of applying a magnetic field to control the magnetization direction of a magnetic body. For example, in an HDD (Hard Disk Drive), the magnetization direction of the medium is reversed by a magnetic field generated by the recording head, thereby performing write. In a conventional MRAM (Magnetic Random Access Memory), a current is supplied to an interconnection provided near the magnetoresistive element, and a thus generated current-induced magnetic field is applied to the cell, thereby controlling the magnetization direction of the cell. Such a current-induced magnetic field writing method that controls the magnetization direction by an external magnetic field has a long history and can be said to be an established technology.
On the other hand, the recent advance of nanotechnology enables remarkable microfabrication of a magnetic material, and the need for locally controlling magnetization on the nanometer scale has arisen. However, localization of a magnetic field is difficult because it characteristically spatially spreads in principle. When selecting a specific storage unit area (bit) or memory cell and controlling its magnetization direction, the smaller the size of the bit or memory cell is, the more conspicuous the problem of “crosstalk”, that is, a magnetic field reaching an adjacent bit or memory cell is. In addition, when the magnetic field generation source is made smaller to localize the magnetic field, the magnetic field enough to control the magnetization direction cannot be generated.
As a technique of solving these problems, a “spin transfer torque method” is known which causes magnetization reversal by supplying a current to a magnetic body (for example, non-patent literature 1).
In this spin transfer torque method, a spin electric current is supplied to a magnetoresistive element as a write current. Magnetization reversal is executed using thus generated spin-polarized electrons. More specifically, the angular momentum of the spin-polarized electrons is transmitted to the electrons in the magnetic material serving as a magnetic recording layer so as to reverse the magnetization of the magnetic recording layer.
Using the spin transfer torque method makes it possible to easily locally control the magnetization state on the nanometer scale and also reduce the value of the spin electric current in accordance with the microfabrication of the magnetic material. This contributes to implementing a spin electronics device such as a hard disk drive or magnetic random access memory having a high recording density.
For example, a magnetic random access memory comprises, as a memory element, a magnetoresistive element having an MTJ (Magnetic Tunnel Junction) film using a TMR (Tunneling Magnetoresistive) effect. The MTJ film includes three thin films, that is, a recording layer and a reference layer which are made of a magnetic material, and a tunnel barrier layer sandwiched between them. Information is stored based on the magnetization states of the recording layer and the reference layer. In a spin transfer torque MRAM using the spin transfer torque method, information write to a magnetoresistive element is done by supplying a current in a direction perpendicular to the surface of the MTJ film.
As the magnetic layers used in the magnetoresistive element, a perpendicular magnetization film in which magnetization occurs in a direction perpendicular to the film surface and an in-plane magnetization film in which magnetization occurs in the in-plane direction are known. When the perpendicular magnetization film is adopted, the leakage magnetic field generated by magnetization of the reference layer is perpendicular to the film surface of the recording layer. Hence, the magnetic field having a large perpendicular component acts on the recording layer. The leakage magnetic field generated from the reference layer and acting on the recording layer acts in a direction to make the magnetization of the recording layer parallel to the magnetization of the reference layer. For this reason, a small spin electric current suffices to reverse the magnetization of the recording layer from antiparallel to parallel. Conversely, a large current is needed to reverse the magnetization from parallel to antiparallel.
Assume that the antiparallel state is unstable because of the leakage magnetic field. In this case, if the leakage magnetic field is larger than the coercive force of the recording layer, it is impossible to hold the magnetization of the magnetoresistive element in the antiparallel state without magnetic field application from outside of the magnetoresistive element. Even if the leakage magnetic field is smaller than the coercive force of the recording layer, the antiparallel state is reversed to the parallel state due to thermal agitation during the antiparallel state maintained for a long time, and the information cannot be held. Hence, the leakage magnetic field from the reference layer needs to be much smaller than the coercive force of the recording layer.
On the other hand, a double junction structure has been proposed in which reference layers are arranged on the upper and lower sides of a recording layer while inserting nonmagnetic layers between them. When the double junction structure is applied to the magnetoresistive element using the perpendicular magnetization films, the magnetization directions of the two reference layers are set to be antiparallel to each other. In this case, the perpendicular components of the leakage magnetic fields generated from the two reference layers and acting on the recording layer face in directions opposite to each other. For this reason, when saturation magnetizations Ms and the thicknesses of the two reference layers are adjusted to be almost the same, the z components (components perpendicular to the film surfaces) of the leakage magnetic fields can almost be canceled.
However, the radial-direction components of the leakage magnetic fields act in directions to strengthen each other. Especially, a large lateral magnetic field acts on the outer periphery of the recording layer. This lateral magnetic field deteriorates the perpendicularity and magnetoresistive characteristics of the magnetization and also degrades the uniformity of magnetization reversal of the recording layer. Additionally, in the double junction structure, the two reference layers need to be antiparallel to each other. To do this, the two reference layers need to have a sufficient coercive force difference and be magnetized separately, resulting in limitations on the degree of freedom of the material, structure, and process conditions.
The magnetoresistive element has a stacked structure in which the recording layer and the reference layer are stacked with the thin tunnel barrier layer sandwiched between them, and the distance between the recording layer and the reference layer is short. When working the stacked structure to form the magnetoresistive element, the recording layer and the reference layer which contain a magnetic metal are removed together with the tunnel barrier layer. Hence, metal readherents may readhere to the side surface of the stacked structure across the tunnel barrier layer. In this case, another leakage current path is formed by the readherents. This causes a short circuit between the recording layer and the reference layer and makes the magnetoresistive element defective, resulting in a decrease in the yield of magnetoresistive elements.
The present applicant has applied a magnetoresistive element in which a magnetic layer is provided on a surface of the reference layer opposite to a tunnel barrier layer so as to cancel the leakage magnetic field from the reference layer. However, since the magnetic layer is very thick, and the height of the stacked film including the reference layer, the tunnel barrier layer, and the recording layer largely increases. For this reason, when patterning the stacked film, the shape of the stacked film may vary, resulting in a variation in the characteristics of the magnetoresistive elements.